This invention relates to spectrophotometric instruments and more particularly, to a spectrophotometric instrument provided with a temperature regulating system, which minimizes the drift in the wavelength and photometric output from the instrument. Spectrophotometers are used to measure color and in the near infrared range, to analyze substances. In such instruments, a broad band of light irradiates a reflecting diffraction grating, which disperses the light into a spectrum. A narrow portion of the spectrum is used to analyze the substance.
The accuracy of the instrument depends upon the capability of the instrument to disperse the parts of the spectrum to precisely predetermined angular positions in the instrument and with precisely predetermined intensities for a given intensity of illumination of the grating. Temperature variation in the instrument causes a position of the spectrum dispersed from the grating to drift as well as causes a drift in the photometric output of the instrument.
Present state-of-the-art instruments mount the components of the instruments on a heat sink, which is cooled by a cooling fan to maintain the temperature of the instrument relatively constant. However, such state-of-the-art instruments are subject to some drift in the spectrum of up to 1/2 of a nanometer in wavelength. To overcome the problem of drift in spectrophotometric instruments, it has been proposed to enclose the instrument in a cast iron case surrounded by heating blankets, the energization of which is controlled by a temperature sensor to attempt to maintain the instrument at a constant temperature. Another system has been proposed to provide oil passageways in the walls of the instrument and to circulate oil through the passageways with the temperature of the oil being regulated. Both of the above described systems involve considerable structural modification of the instrument and would substantially add to the cost of the instrument.
The present invention achieves temperature control of the instrument by a very simple expedient. In the instrument of the present invention the components of the instrument are mounted on a thick central heat conducting plate, which serves as a heat sink. A fan is provided, which directs air over both sides of this plate. In accordance with the invention, the speed of the fan is controlled through a continuous speed range in accordance with the output signal of a temperature sensor mounted to sense the temperature of the heat sink to maintain the temperature of the heat sink at a constant temperature. With this system, the temperature of the heat sink plate is held at a substantially constant temperature about 10 minutes after start up of the instrument and the photometric drift is held to a very low value after 10 minutes. The wavelength drift in the instrument after the instrument has been turned on for 85 minutes drops to less than 0.04 nanometers. Thus, an extraordinary drop in the wavelength drift is achieved with a very simple temperature control system.